Abstract

Plasma has a data abstraction mechanism called DataEngines. You can ask for a DataEngine by name, request a source from it (again, by name) and you get data back from it, either at fixed intervals or whenever it is updated by the DataEngine internally. The data is a map (which is the same as an object in JavaScript) of string keys to data.

There are two new things in this plasmoid. We've implemented plasmoid.dataUpdate, and connected a data engine source to it.

If you connect a data engine source to object, when there is an update for that source the method object.dataUpdate is called with two arguments: the name of the source and a map containing the data provided by the source.

In this case, plasmoid.dataUpdate gets called with "Local" as the first argument and the following object as the second argument (on my machine right now):
{

The last line of the script does the actual connection. We select the "time" data engine and request the "Local" source from it, which provide the local time. We tell it to connect to the plasmoid object and force an update every 500ms (every half a second).

If we left off the last parameter to connectSource (ie: didn't provide an update interval), then the source would be updated when there was new data available. This doesn't make much sense for a clock, where the time is continually changing, so would actually get no updates at all if you did this. However, there are other data engines (such as the "soliddevice" engine) where you only want to be notified when something changed, and in this case you would leave off the update interval.

Connecting Engines to Plasma Interface Elements

A number of Plasma interface elements support connecting DataEngines directly to them. These include:

Label

Meter

TextEdit

With these widgets, one can simply direct the DataEngine to update the widget directly and it will attempt to display the data. So in our example we could also write it as:

layout = new LinearLayout(plasmoid);

label = new Label();
layout.addItem(label);

plasmoid.dataEngine("time").connectSource("Local", label, 500);

While this is not as flexible as implementing plasmoid.dataUpdate, it can be a useful technique.

Pro tip: plasmaengineexplorer

To find out what engines are available and what sources they provide, run
plasmaengineexplorer

You can choose from any of the installed data engines and request a source, including setting an update interval. Just remember that when you are reading the data in JavaScript, spaces in the keys provided by the sources are replaced by underscores. So each source in the time engine provides a "Timezone Continent" key, but in JavaScript you would access that as data.Timezone_Continent or data["Timezone_Continent"].